Indexing dithering mechanism and method

ABSTRACT

Dithering mechanism and method for eliminating the effects of zero-rate bias in a rate sensor or gyroscope. Both continuously moving and indexing embodiments are disclosed. The mechanism includes a first part mounted in a fixed position centered about a dither axis perpendicular to the input axis of the gyroscope, a second part disposed coaxially of the first part and affixed to the sensing element of the gyroscope, and a plurality of piezoelectrically driven quartz flexure beams extending radially between the first and second parts for dithering the second part about the dither axis. In some embodiments, the dithering mechanism is formed separately from and affixed to the sensing element of the gyroscope, and in others it is formed integrally with the sensing element. In the indexing embodiments, radial arms and fixed stops limit movement of the mechanism between two fixed positions, and drive signals and holding potentials are applied alternately to dither the mechanism between the two positions and to hold it alternately in those positions during successive data acquisition periods.

BACKGROUND OF THE INVENTION

1. Field of Invention

This invention pertains generally to angular rate sensors or gyroscopesand, more particularly, to a dithering mechanism and method foreliminating the effects of zero-rate bias in a rate sensor or gyroscope.

2. Related Art

Angular rate sensors or gyroscopes exhibit a non-zero output in theabsence of rotation about the input axis. This non-zero output is calledbias, and it can cause significant errors in inertial measurements. Atthe time of manufacture, the bias for each individual gyroscope can bemeasured and subtracted from the output to set the zero-rate output tozero. However, bias is not fixed in time, and it tends to drift withchanges such as temperature and aging of the materials employed in thesensor. This may require periodic re-zeroing of a sensor in the field.

Bias cancellation, or “washout”, mechanisms are employed in order toreduce or eliminate the effects of biases that slowly change with timein an unpredictable manner. By dithering the input, or sensitive, axisof a gyroscope, such mechanisms are able to provide automatic adjustmentfor changes in zero-rate output due to temperature, time or othererrors.

Dithering of gyroscopes is well known in the art. A ring lasergyroscope, for example, is dithered around its sensitive axis for thepurpose of avoiding frequency lock-in. However, rotating a sensor aboutits input axis is undesirable because it provides an angular rate inputother than the one to be detected.

Dithering the input axis of a gyroscope about an axis perpendicular tothe input axis to cancel bias is also well known. Continuously rotatingthe input axis to eliminate the effects of bias is known as carouseling, and periodically moving the input axis between discrete locations toremove or cancel bias is known as indexing.

OBJECTS AND SUMMARY OF THE INVENTION

It is in general an object of the invention to provide a new andimproved dithering mechanism and method for eliminating the effects ofzero-rate bias in a gyroscope.

Another object of the invention is to provide a dithering mechanism andmethod of the above character wherein the mechanism is fabricated bymicromachining techniques.

These and other objects are achieved in accordance with the invention byproviding a dithering mechanism and method in which a first part ismounted in a fixed position centered about a dither axis, a second partis disposed coaxially of the first part and affixed to the sensingelement; and a plurality of piezoelectrically driven flexure beamsextend radially between the first and second parts. A drive signal isapplied to the flexure beams to rotate the sensing element toward afirst fixed position, a first electrostatic holding potential retainsthe sensing element in the first fixed position during a first dataacquisition period, and at the end of the first data acquisition periodthe first electrostatic holding potential is removed and another drivesignal is applied to the flexure beams to assist the flexure beams inrotating the sensing element back toward a second fixed position. Asecond electrostatic holding potential retains the sensing element inthe second fixed position during a second data acquisition period. Thedrive signals and holding potentials continue to be applied alternatelyto swing the sensing element back and forth and hold it in alternateones of the two fixed positions during successive data acquisitionperiods.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top plan view of one embodiment of a dithering mechanismincorporating the invention.

FIG. 2 is a top plan view of one embodiment of a tuning fork mounted onthe dithering mechanism of FIG. 1.

FIG. 3 is a fragmentary bottom plan view of the embodiment of FIG. 2.

FIG. 4 is a vertical sectional view of the embodiment of FIG. 2.

FIG. 5 is a top plan view of another embodiment of a tuning fork with adithering mechanism incorporating the invention.

FIG. 6 is a vertical sectional view of the embodiment of FIG. 5.

FIG. 7 is a vertical sectional view of another embodiment of a tuningfork with a dithering mechanism incorporating the invention.

FIG. 8 is a fragmentary bottom plan view of another embodiment of atuning fork with a dithering mechanism incorporating the invention.

FIG. 9 is a top plan view of the dithering mechanism in the embodimentof FIG. 8.

FIG. 10 is an enlarged, fragmentary, horizontal sectional view of thedithering mechanism in the embodiment of FIG. 9.

FIG. 11 is a vertical sectional view of the embodiment of FIG. 8.

FIG. 12 is a graphical representation of the dithering motion of thetuning fork in the embodiment of FIG. 8.

FIG. 13 is a graphical representation of the drive voltage applied tothe piezoelectrically driven quartz flexure beams of the ditheringmechanism in the embodiment of FIG. 8.

FIGS. 14 and 15 are graphical representations of the electrostaticholding potentials applied respectively to the clockwise andcounter-clockwise stops of the dithering mechanism in the embodiment ofFIG. 8.

DETAILED DESCRIPTION

As illustrated in FIG. 1, the dithering mechanism includes a mountingpost 11 and an outer ring 12 which is disposed coaxially about the post.Both the post and the ring are hexagonal in shape and centered about adither axis 13, with the post in a hexagonal opening 14 in the ring. Thering is mounted to the post by radially extending flexure beams 16, andmounting pads 17 are provided on the upper side of the ring forattachment to the sensing element of a gyroscope.

The dithering mechanism is formed as a unitary structure of apiezoelectric material such as crystalline quartz and fabricated bymicromachining techniques such as photo lithography, etching and surfacedeposition. The flexure beams are driven as piezoelectric actuators bysignals applied to electrodes (not shown) mounted on the beams to ditherthe ring through a small angle, typically on the order of 10 degrees. Inorder to maintain a stable angle of dither, the drive signals preferablyhave an amplitude which varies constantly and continuously such as thatof a pure sinusoidal waveform. A suitable sinusoidal drive signal can,for example, have an amplitude on the order of 100 volts or less and afrequency on the order of 10 to 100 Hz.

Thus, the flexures function as piezoelectric actuators as well assuspension beams. In addition, electrical conductors or traces can bemounted on them for carrying the drive signals to the actuators anddrive and pick-up signals for the sensing element dithered by themechanism.

The hexagonal structure of the mechanism is particularly compatible withthe trigonal crystal symmetry of quartz, and the six flexural memberscan be aligned along axes which provide optimal piezoelectric couplingfor actuating the dithering motion, e.g., along either thecrystallographic x-axis or the crystallographic y-axis of the quartz.

If desired, the mechanism can be fabricated of a material other thanquartz. Other suitable materials include other piezoelectric crystalssuch as lithium niobate or lithium tantalate, piezoelectric ceramicssuch as lead zirconium titanate (PZT), and thin-film piezoelectriccoatings such as PZT film on single-crystal silicon.

In FIGS. 2-4, the dithering mechanism is illustrated in conjunction witha sensing element in the form of a tuning fork 19. The tuning fork has apair of drive tines 21, 22 and a pair of pickup tines 23, 24 whichextend in opposite directions from a central body or base 26 and aredisposed symmetrically about the input axis 27 of the device. The bodyincludes a frame 28 which surrounds a central opening 29, with amounting pad or base 31 within the opening connected to the frame byrelatively thin bridges 32. Like the dithering mechanism, the tuningfork is formed as a unitary structure of a piezoelectric material suchas quartz. Drive and pickup electrodes (not shown) are mounted on thetines in a conventional manner.

The tuning fork is mounted on the dithering mechanism, with the base 31of the tuning fork resting on mounting pads 17 and the lower end of post11 affixed to the base 33 of the package in which the gyroscope ishoused. Base 31 is bonded to mounting pads 17 and post 11 is affixed tothe package by suitable means such as solder, epoxy or other adhesive.Dither axis 13 is perpendicular to the input or sensitive axis 27, ofthe gyroscope, and dithering of ring 11 is transferred to the base ofthe tuning fork so that the input or sensitive axis of the tuning forkis dithered about the dither axis.

In the embodiment of FIG. 5, the dithering mechanism is formedintegrally with the tuning fork. In this embodiment, the tuning fork andthe dithering mechanism are formed as a unitary structure of crystallinequartz, with a hexagonal mounting post 34 disposed in a hexagonalopening 35 in the central body 36 of the tuning fork. The tuning fork 37is suspended from the mounting post by flexure beams 38 which aresimilar to flexure beams 16. The post extends below the under side ofthe tuning fork and is affixed to the package 39 in which the gyroscopeis housed.

As in the embodiment of FIG. 2, the tuning fork has drive tines 41, 42and pickup tines 43, 44 which extend in opposite directions from centralbody 36 and are disposed symmetrically about an input axis 46. Whendriven piezoelectrically, flexure beams 38 cause the tuning fork todither about the axis 47 of mounting post 34 which is perpendicular toinput axis 46.

In this embodiment, the flexures once again function both aspiezoelectric actuators and as suspension beams, and electricalconductors or traces can be mounted on them for carrying the drivesignals to the actuators as well as for carrying drive and pick-upsignals to and from the electrodes on the tines of the tuning fork.

Means is provided for monitoring the instantaneous rotational positionof the tuning fork relative to the base on which it is mounted, and inthe embodiment illustrated, the scale 48 of an optical encoder isapplied to the upper surface of the tuning fork and disposedconcentrically of dither axis 47. Alternatively, another suitable typeof encoder, such as a capacitive encoder, can be utilized, if desired.

The embodiment of FIG. 7 is generally similar to the embodiment of FIG.5, and like reference numerals designate corresponding elements in thetwo embodiments. In this embodiment, however, a second tuning fork 51 ispositioned beneath tuning fork 37. Tuning fork 51 is identical to tuningfork 37 and is stacked congruently with it, with mounting post 34extending through a central opening 52 and flexure beams 53 mounting thefork to the post. The flexure beams for the two tuning forks are drivenout of phase so that the tuning forks dither in an anti-phase mannerabout dither axis 47. With a common dither frequency and with equal andopposite angular displacements, the net angular momentum is zero, andlittle or no vibration is coupled to the package. If desired, a dummymass can be utilized instead of the second tuning fork and still providea balancing of angular momentum within the system.

With the embodiments described thus far, the tuning fork swingscontinuously back and forth through a small angle. In the embodiment ofFIG. 8, however, the tuning fork is periodically swung between andlocked in two predetermined positions for data acquisition. As in theembodiment of FIG. 1, the dithering mechanism is formed as a unitarystructure of a piezoelectric material such as crystalline quartz with asubstrate 54, a mounting post 56 affixed to the substrate, an outer ring57 disposed coaxially of the post, and flexure beams 58 extendingradially between the post and the ring. The post is mounted in a fixedposition, and a tuning fork 59 is mounted on the ring by mounting pads61 for movement about the axis 62 of the post. Post 56 and ring 57 arehexagonal in shape, and flexure beams 58 are driven as piezoelectricactuators by signals applied to electrodes (not shown) on the beams.

Arms 63 extend out in radial directions from the corners or vertices ofthe ring 57, and stops 64 are mounted in fixed positions on oppositesides of the arms to limit rotation of the ring. The confrontingsurfaces of the arms and the stops intercept a small angle, typicallyabout 10 degrees, when the ring is in a neutral position.

Electrodes 66, 67 are mounted on the confronting faces of arms 63 andstops 64 and energized to produce electrostatic forces for locking thering in the two extreme positions with the arms abutting against thestops. Small protrusions 68 on the stops extend through openings inelectrodes 67 and abut against electrodes 66 to prevent the electrodesfrom contacting each other and shorting out. The length of theprotrusions determines the width of the gaps between the electrodes whenthe mechanism is in the two locked positions, and with current photolithography technology, the protrusions can have a length on the orderof 2 microns. Since the thickness of the electrodes is negligiblecompared to that dimension, the width of the gaps is substantially equalto the length of the protrusions.

The movement of the dithering mechanism and the tuning fork mountedthereon is illustrated in FIG. 12, where the rotational position of themechanism is shown as a function of time. This figure shows a start-upphase which occurs during the interval between 0 and 1 unit of time, afirst data acquisition phase during the interval between 1 and 2 unitsof time, a second data acquisition phase during the interval between 2.1and 3 units of time, and a third data acquisition phase during theinterval between 3.1 and 4 units of time.

Because of the relatively high Q of the dither-gyro system (Q>1,000) ittakes time to pump enough energy into oscillatory system in order toreach the desired amplitude of oscillation. This process takes placeduring the start-up phase and manifests itself as a rise of amplitude ofoscillation.

As illustrated in FIGS. 12-14, when the amplitude of the oscillationreaches the distance between the stops, the drive voltage is removedfrom piezoelectric actuators 58, and an electro-static locking potentialis applied to the electrodes 66 on the clockwise faces of arms 63 and tothe electrodes 67 on the counter-clockwise faces of stops 64. Thelocking potential is maintained during the first data acquisitioninterval, and the mechanism and tuning fork are thus held in theclockwise position during that interval.

At the end of the first data acquisition phase, the first lockingpotential is removed, and energy stored in the flexure beams oractuators swings the dithering mechanism back in the counter-clockwisedirection. A small amount of mechanical energy is lost in the system,and a drive pulse is applied to the piezoelectric actuators, as shown inFIG. 13, to ensure that the mechanism swings to the fullcounter-clockwise position. When that position is reached, a secondholding potential is applied to the electrodes on the counter-clockwisefaces of arms 63 and to the electrodes on the clockwise faces of stops64, as shown in FIG. 15, to lock the mechanism in the counter-clockwiseposition. This locking potential is maintained during throughout thesecond data acquisition interval, and the mechanism and tuning fork arethus held in the counter-clockwise position during that interval.

At the end of the second data acquisition phase, the second lockingpotential is removed, and the dithering mechanism swings back in theclockwise direction. Another drive pulse is applied to the piezoelectricactuators to ensure that the mechanism swings to the full clockwiseposition. When that position is reached, the first holding potential isonce again applied to the electrodes on the counter-clockwise faces ofarms 63 and to the electrodes on the clockwise faces of stops 64 to lockthe mechanism in the counter-clockwise position throughout the thirddata acquisition interval.

This process continues, with the tuning fork being held first in onedata acquisition position and then in the other.

For a given mechanism, the required electrostatic holding potential canbe determined from the following equation for balancing theelectrostatic attraction force (on the left) and the elastic restoringforce (on the right):

${\frac{ɛ_{0}{AV}^{2}}{2g^{2}} = {kg}},$where ∈₀ is the dielectric constant, A is the area of the electrodes, Vis the electrostatic locking potential, g is the gap in the lockedposition, and k is the elastic constant of the flexure beams.

Thus, for example, with a crystalline quartz structure, flexure beamshaving a thickness on the order of 50 microns and a length on the orderof 19 mm, and electrodes having dimensions on the order of 625 microns×2mm, and gaps of approximately 2 microns, the holding potential requiredis on the order of 0.6 volt.

The advantage of piezoelectric actuation, dynamic switching, andelectro-static locking is readily apparent when the relatively low 0.6 Vholding potential is compared with the actuating potential that would berequired for a dithering mechanism of similar dimensions without thepiezoelectric actuators and dynamic switching. In such a mechanism, theinitial gaps between arms 63 and stops 64 (the gaps when the arms are intheir neutral positions between the stops) are on the order of 2.5 mm.With electrostatic pull-in occurring when the gaps are about two-thirdsof the initial gaps, i.e. about 1.7 mm, the potential required to drawthe arms to the stops would be on the order of 13.3 kV, a highlyimpractical potential compared to 0.6 volt.

Although the invention has been disclosed in connection with a ditheringmechanism which is particularly suitable for zero-rate biascancellation, other applications are possible as well. By applying astatic voltage to the mechanism, the input axis of the gyroscope can bevaried to provide a fine adjustment for pointing the gyroscope. This maybe done to achieve a more accurate position of the input axis than canbe achieved by just the mechanical assembly of the device. In addition,the orientation of the input axis can be varied to adjust for variationsin temperature or mechanical drift over time, or it can be adjusted tonew positions as required by the needs of the system in which it isused.

It is apparent from the foregoing that a new and improved ditheringmechanism and method have been provided. While only certain presentlypreferred embodiments have been described in detail, as will be apparentto those familiar with the art, certain changes and modifications can bemade without departing from the scope of the invention as defined by thefollowing claims.

1. A dithering mechanism for a gyroscope having a sensing element formonitoring rotation about an input axis, comprising: a first partmounted in a fixed position centered about a dither axis, a second partdisposed coaxially of the first part and affixed to the sensing element,a plurality of piezoelectrically driven flexure beams extending radiallybetween the first and second parts for dithering the second part aboutthe dither axis, an arm extending from the second part and a pair ofstops on opposite sides of the arm for limiting rotation of the secondpart between first and second fixed positions, and electrodes on the armand the stops to which electrostatic holding potentials are applied tohold the second part in the first and second fixed positions duringsuccessive data acquisition periods.
 2. The dithering mechanism of claim1 wherein the first part, the second part and the flexure beams areformed as a unitary structure of crystalline quartz.
 3. The ditheringmechanism of claim 2 wherein at least one of the flexure beams isaligned along a crystallographic axis of the quartz.
 4. The ditheringmechanism of claim 1 wherein the first part is disposed coaxially withinthe second part.
 5. The dithering mechanism of claim 1 wherein the firstpart is a hexagonal post, the second part is a hexagonal ring, and sixflexure beams extend between the post and the ring.
 6. The ditheringmechanism of claim 1 wherein the second part is a hexagonal ring, sixarms extend radially from corners of the hexagonal ring, and stops arepositioned on opposite sides of each of the arms.
 7. The ditheringmechanism of claim 1 wherein the dither axis is perpendicular to theinput axis.
 8. A method of dithering the sensing element of a gyroscopewith a mechanism having a first part mounted in a fixed positioncentered about a dither axis, a second part disposed coaxially of thefirst part and affixed to the sensing element; and a plurality ofpiezoelectrically driven flexure beams extending radially between thefirst and second parts, comprising the steps of: applying a drive signalto the flexure beams to rotate the sensing element toward a first fixedposition, applying a first electrostatic holding potential to retain thesensing element in the first fixed position during a first dataacquisition period, removing the first electrostatic holding potentialat the end of the first data acquisition period, applying another drivesignal to the flexure beams to assist the flexure beams in rotating thesensing element back toward a second fixed position, and applying asecond electrostatic holding potential to retain the sensing element inthe second fixed position during a second data acquisition period. 9.The method of claim 8 wherein the first an second fixed positions aredefined by an arm extending from the second part and fixed stops onopposite sides of the arm.
 10. The method of claim 9 wherein theelectrostatic holding potentials are applied to electrodes mounted onthe arm and on the stops.
 11. The method of claim 8 wherein theadditional drive signal is a voltage pulse.
 12. The method of claim 8wherein the dither axis is perpendicular to an input axis about whichrotation is sensed.
 13. A method of dithering the sensing element of agyroscope about a dither axis perpendicular to an input axis about whichrotation is monitored, comprising the steps of: (a) mounting a firstpart in a fixed position centered about the dither axis; (b) positioninga second part coaxially of the first part; (c) connecting the secondpart to the first part with a plurality of radially extendingpiezoelectrically excitable flexure beams; (d) affixing the sensingelement to the second part; (e) applying a drive signal to the flexurebeams to rotate the sensing element toward a first fixed position: (f)applying a first electrostatic holding potential to retain the sensingelement in the first fixed position during a first data acquisitionperiod; (g) removing the first electrostatic holding potential at theend of the first data acquisition period; (h) applying another drivesignal to the flexure beams to assist the flexure beams in rotating thesensing element back toward a second fixed position; (I) applying asecond electrostatic holding potential to retain the sensing element inthe second fixed position during a second data acquisition period; and(j) repeating steps (e)-(i) to swing the sensing element back and forthand hold it in alternate ones of the two fixed positions duringsuccessive data acquisition periods.
 14. The method of claim 13 whereinthe first an second fixed positions are defined by an arm extending fromthe second part and fixed stops on opposite sides of the arm.
 15. Themethod of claim 14 wherein the electrostatic holding potentials areapplied to electrodes mounted on the arm and on the stops.
 16. Themethod of claim 13 wherein the additional drive signal is a voltagepulse.